Assessment of Th-232 nuclear data through analysis of thorium-loaded critical experiments in thermal-neutron systems using the Kyoto University Critical Assembly

The main purpose of this study is to make an assessment of Th-232 nuclear d ata through the analysis of thorium-loaded critical experiments in thermal neutron systems using the Kyoto University Critical Assembly (KUCA). The im pact of the difference among the nuclear data libraries of Th-232 was exami ned through neutronics calculations. Using JENDL-3.2, k(eff) for thorium -l oaded KUCA cores were overestimated by 0.9% to 1.2%. Although approximately 0.7% of the overestimation is due to the U-235 cross section used in the d river region, the overestimation for thorium-loaded cores is apparently lar ger than those for cores free of thorium by about 0.2% to 0.5%. This overes timation depends on the neutron spectrum, and becomes larger for hard-spect rum cores. The Use of Th-232 cross sections from ENDF/B-VI or JEF2.2 also l ead to overestimation of k(eff). The overall trend of the overestimation by ENDF/B-VI and JEF2.2 is the opposite of that by JENDL-3.2, and becomes sma ller in hard-spectrum cores. The reactivity difference caused by different Th-232 evaluations reaches up to -0.5% in hard-spectrum cores. Those signif icant discrepancies in reactivity of thorium-loaded thermal systems, caused by the difference of Th-232 cross sections compiled in different nuclear d ata libraries, are mainly due to the difference in the capture cross sectio n at thermal region, lower resonance and unresolved resonance regions, and also due to the difference in scattering cross section at MeV region. It be came clear that none of the current Th-232 evaluations can accurately simul ate the criticality of the thorium-loaded KUCA experiments. These facts ind icate that there remains a certain ambiguity in the evaluated Th-232 cross sections, which is desirable to be eliminated for further design studies of thorium-based fuel cycle.